1. Field of the Invention
The present invention relates to a vehicle steering system including a angle variable mechanism for changing an angle ratio of a steering angle of a steering member to a turning angle of a steering mechanism for turning left and right steered wheels.
2. Related Art
A variable angle ratio mechanism included in a vehicle steering system is made up of a first sun gear which is connected to a steering member such as a steering wheel, a second sun gear which is disposed coaxially with the first sun gear and is connected to a steering mechanism, a first planetary gear which meshes with the first sun gear, a second planetary gear which meshes with the second sun gear and rotates together with the first planetary gear, a carrier which supports the first and second planetary gears in such a manner as to freely rotate about their own axes and revolve around the sun gears, and an electric motor for rotating the carrier, and is configured to rotate the second sun gear via the first sun gear, the first planetary gear and the second planetary gear and also to rotate the second sun gear at increased speeds via the carrier by virtue of driving by the electric motor, so as to change the angle ratio of steering angle to turning angle (for example, JP-A-2005-343205). In addition, a connecting shaft is press fitted in shaft holes opened in their rotational centers of the first and second planetary gears for integral connection thereof.
There is known a vehicle steering system which includes such a variable angle ratio mechanism in which one ends of the first and second planetary gears which are formed by cutting are butted against each other, and the butted portion is welded so as to integrally connect the first and second planetary gears together. FIGS. 4A to 4D are explanatory drawings which show steps of connecting first and second planetary gears 100, 101 through welding. Gear wheel materials 100a, 101a each having a large diameter portion and a small diameter portion which continuously connects to the large diameter portion are molded by molds (FIG. 4A), teeth are cut into the large diameter portions of the gear wheel materials 100a, 101a by a hobbing machine (FIG. 4B), following this, a heat treatment such as quenching is applied thereto, then, shaft holes are opened in central portions of the gear wheel materials (FIG. 4C), the small diameter portions of two resulting planetary gears 100, 101 are butted against each other after tooth faces thereof are ground, and the butted portion is welded (FIG. 4D). Then, a shaft element is press fitted in the shaft holes of the respective planetary gears 100, 101.
In a variable angle ratio mechanism in which first and second planetary gears are connected together by a connecting shaft as done in JP-A-2005-343205, however, teeth cutting work needs to be performed in a separate step, and moreover, a circumferential phase matching between the first and second planetary gears is difficult, there having been a demand for improvement in this respect.
In addition, in a variable angle ratio mechanism in which two planetary gears are connected together through welding, since there are necessary the steps of molding two gear wheel materials, cutting teeth in the gear wheel materials, applying a heat treatment to the same, opening shaft holes therein, grinding them and welding together the resulting gear wheels, the number of machining steps is increased, leading to a problem that the production costs of planetary gears and hence a vehicle steering system are increased. In addition, strain is generated in the connecting portion due to heat being applied to the planetary gears which is generated in association with welding, which causes a circumferential phase deviation between the first and second planetary gears, leading to a problem of increasing a difference between a backlash amount at a meshing portion between the first sun gear and the first planetary gear and a backlash amount at a meshing portion between the second sun gear and the second planetary gear. In addition, since the small diameter portions of the first and second planetary gears need to be formed relatively long so as to secure a space necessary for welding, the overall length in the tooth face width direction of the planetary gear is increased, leading to a problem the variable angle ratio mechanism has to be made large in size.